Micrometeorites from Rhodes Bluff, West Antarctica

Micrometeorites account for the largest influx of extraterrestrial particles into Earth and can be recovered from almost every corner of the planet. In the past few decades, pristine collections have been retrieved from high-altitude sediments from Antarctica. Here, we report a new collection of particles that have been recovered near Rhodes Bluff, West Antarctica, and describe the physicochemical properties of this new collection. First, the sediment is sieved into different size fractions, after which magnetic particles are separated using a neodymium magnet. Then, the particles are individually picked under an optical microscope, from which and more than 600 potential micrometeorite particles were recovered. MicroX-ray fluorescence (Bruker, Tornado M4) is used as the first tool to distinguish extraterrestrial particles from local background material using major and minor element compositions, which resulted in 270 possible micrometeorite candidates. Then, A first geochemical and textural classification is done using SEM-EDS. In total, 137 well-preserved micrometeorites have been recovered from a total of 1.5 kg of sediments. An unusual high abundance of  I-type cosmic spherules, as well as scoriaceous and unmelted, is observed. The I-type accounts for 12.4% by number (17 micrometeorites) of the 137 particles, while the combined scoriaceous and unmelted particles make up 11.6% (16 micrometeorites). This high abundance of I-type is quite rare in the Antarctica Collection and that of the scoriaceous together with unmelted particles is unique to this collection, an extensive study is required to produce the exact root cause.. From calculating the size of the particles, a size frequency distribution (Fig. 1) shows a size frequency peak distinct from other urban and Antarctic collections. A distinct accumulation or preservation mechanism may need to be invoked, or alternatively a distinct exposure history or source material may have influenced the Rhodes Bluff collection. Apart from this distinct peak on the size frequency plot, four particles with unusually high sulfur and iron content have also been found (Fig. 2). The preliminary results point towards a common extraterrestrial origin of these particles, and may trace the arrival of unique (micro)meteoritic material to Earth. Further studies with EPMA or SIMS will aid in understanding parent body source in the solar system, and weathering effects of the spherules.

References:

[1]. Genge et al., (2018), Meteoritics & Planetary Science 53, Nr 10, 2051–2066.

[2]. Suttle and Folco., (2020), Geophysical Research: Planets, 125, e2019JE006241.

[3]. Suttle et al., (2021),  Meteoritics & Planetary Science 56, Nr 8, 1531–1555

Fig.1 : Size distribution of various micrometeorite collections. Data from Rhodes Bluff, West Antarctica is compared with collections by Genge et al., 2018 (Larkman Nunatak), Suttle and Folco 2020 (Transantarctic Mountains, TAM65) and Suttle et al., 2021 (Urban). The urban and Larkman Nunatak collections show a comparable size frequency peak at 160 µm, while the TAM65 is dominated by a size frequency peak at 250 µm. In the case of the current study, a major peak occurs around ~90 µm, while a secondary peak is observed at ~180 µm.

Fig. 2: Examples of extracted extraterrestrial particles with a high sulfur and iron content, which shows a porphyritic to I-type texture. The particles have been analyzed using BSE-SEM-EDS. The SO2 content of the particles ranges from 5.2 – 29.4 wt% and the FeO content ranges from 48.7 – 70.8 wt%.